DETERMINING OPTIMUM GEOMETRY FOR A BOW THRUSTER PROPELLER

Transcription

1 DEERMINING OPIMUM GEOMERY FOR A BOW HRUSER PROPELLER Y.H. OZDEMIR, S. BAYRAKAR,. YILMAZ, M.GUNER Yildiz echnical University, Dept. of Naval Architecture and Marine Engineering, Besiktas, Istanbul, URKEY SUMMARY In this study an optimum geometry for a bow thruster propeller is obtained by computational fluid dynamics (CFD) and lifting line theory. Commercially available finite volume CFD code of FLUEN was used to calculate threedimensional, incompressible, steady-state continuity and momentum equations. Reynolds-Averaged Navier-Stokes (RANS) solver was chosen for modeling turbulence. urbulence closure is obtained using standard k-epsilon model with standard wall function. It can be concluded from this work that the present tool gives an easy approach for design and analyzing a bow thruster before the manufacture 15. INRODUCION here are two types of driving systems to improve the maneuvering capability of the ships. hese are: Propeller system that located in a tunnel that opened in the aft or fore of the ship (bow or stern driving systems), Water jet located in the bottom of the ship or vector driving systems. Driving systems that located in bow or stern have been used to change the direction of the ship at the low speed for maneuvering. Generally at about 3 knots the driving force that supplied from side drivers decreases to 1.5 knots and disappears completely at 5 knots. For such drivers the moving direction can be changed by reversing the direction of the propellers. he power of the bow/stern thrusters depends on the driving force. For a simple comparison, the required driving force per water line lateral area for different ship types is given in able 1. Required driving power for a bow thruster can be calculated [1] empirically as given in Eq.1. 3 P = k 2 / kw (1) In equation 1, k is the coefficient that depends on the ship type as depicted in able 2. able 1 Required driving force per water line lateral area for different ship types [1]. Ship type Driving force required for per water side area 2 [kn/mp P] anker / Bulk carrier Container Ferry With the recent technological developments, high speed computing technologies and reliable computational fluid dynamics (CFD) techniques lead to the increasing use of CFD for the solution of naval architecture and marine engineering problems. Seakeeping and vessel boundary layers, free surface flow [2], propeller performance [3], etc are the main scope of the CFD in the marine industry. able 2 Required coefficient of k to calculate bow thruster driving force for different ship types [1]. Ship ype k anker / Bulk Carrier 0.60 Container 0.70 Ferry 1.35 Hoffmann et al. [4] performed series of cavitations tunnels tests obtained velocity fields were then compared with CFD analysis of a working propeller. Effect of the wake at propeller plane was reported by Subramanian and Vijayakumar [5]. It was concluded that reduced rake angle at the stern improves the wake fraction and using surface generation tools the inverse design process could be adapted to obtain favorable flow conditions. Numerical calculation of open water characteristics of a highly skewed propeller was analyzed using RANS

2 method by Da-Qing [6]. hrust and torque coefficients were obtained for advance ratio of A special attention was to the grid structure, as it was reported that flow field could not fully resolved on a coarse grid. Celik [7] investigated effects of the wake equalizing duct (WED) on the propulsion performance for several arrangements of WED for a number of ship speeds. It was emphasized that propeller characteristics and resistance of the ship were slightly affected by the presence of WED. 16. MAHEMAICAL MODEL In this paper, design and CFD analysis of a bow thruster with 200 kw is presented. At the design stage optimum revolution of the propeller and optimum blade geometry was determined. he main procedures followed are given as follows: he geometry of the propeller was obtained using lifting line [8] method where the blade area is selected to minimise the cavitation risk. A simple beam theory stressing procedure is used to calculate Figure 2: he developed geometry of the bow thruster with 200 kw. Celik and Guner [8] also reported a detailed procedure for the design of marine screw propellers and hydrodynamic performance of the combined propeller, which was analyzed with lifting line theory. A numerical analysis of propeller hydrodynamic performance was reported by Gu and Kinnas [9]. Contrary to Celik and Guner, they emphasized the contra-rotating and ducted propellers. he effects of the duct on the impeller were concluded. Figure 1: ypical bow thruster configuration [1]. he calculation of propeller induced pressure pulses was carried out by a quasi continuous Vortex-Lattice method by Stoye et al [10]. Performance of the CFD simulation of the flow past a marine propeller was concluded by Sileo et al. [11]. hrust and torque coefficients were calculated for the conditions related to 1500 rpm and different values of advance ratios. As reported earlier, it was issued that mesh structure should be fine enough to resolve the flow and to obtain correct global parameters. the blade section thickness. he blade section is symmetric in thickness and chord and diameter developed from circular arcs. he performance of the designed propeller was analyzed with Lifting Surface heory and CFD technique. Bow thruster propeller was designed with 3-blade and its diameter was chose as 0.8 m. he working revolution of the propeller was considered as 800 rpm. Based on these pre-defined parameters, the geometry of the designed propeller is given in Figure 2. After the pre-designed procedure obtained, bow thruster propeller geometry was analyzed with a commercially available CFD package of FLUEN and a computer code that has been developed in Yildiz echnical University s (YU) Department of Naval Architecture and Marine Engineering. his computer code uses lifting surface. All mentioned packages provide deep insight into the cavitations, engine power relation, and other hydrodynamics values such as driving, section surface pressure distribution, etc. An optimization of the bow thruster geometry was done based on the output of these analyses. hree-dimensional (3-D) geometry of the obtained geometry generated in a CAD package of RHINO is shown in Figure 3, which depicts however, propeller that was produced. he first one was changed to avoid cavitations phenomena. he present CFD simulation is performed on a threeblade propeller where D=0.8m. For computational investigation the propeller was located in a domain which is given in Figure 4. Based on the pre-design by using a computer code developed in YU, obtained

3 geometry was analyzed with CFD code of FLUEN Simulations are performed by taking into account the two rotation of the blade. As can be seen from Figure 4, the computational domain consist of a cylinder that propeller was located in for analyses. otal length of the cylinder is three times of the propeller diameter. Figure 4: Computational domain. (a) (a) (b) Figure 3: General design (a) and bow thruster geometry that appropriate to the existed core length. All the grid structure was generated by using GAMBI a pre-processor of FLUEN. otally 3,500,000 hexahedral mesh elements are used for the whole domain. Figure 5a shows the mesh structure for the computational domain while meshed bow thruster was shown in Figure 5b. Boundary conditions were set to simulate flow around the rotating propeller in open water. Pressure inlet and pressure outlet was specified as inlet and outlet conditions. (b) Figure 5: Generated mesh structure for the bow thruster (a) and whole domain (b). he present computations are conducted on a rotating frame which is fixed on the propeller blades. In fact, when the domain rotates at a constant angular velocity the analysis can be simplified if the problem is analyzed in a coordinate system which rotates together with this domain. he solver considered in the present simulation uses finite volume method as a discretization scheme. he flow is taken as steady, incompressible and threedimensional. In order to calculate the pressure field and

4 and are to couple it properly to the velocity field, a pressurecorrection method of SIMPLE type segregated algorithm adapted to unstructured grid was used. As turbulence closure Standard k-ε (SKE) turbulence model with standard wall-function is used. he k-ε turbulence model is one of several two-equation models that have been developed over the years. It is probably the most widely and thoroughly tested of them all. As it is well known that SKE is a semi-empirical model based on model transport equations for the turbulence kinetic energy, k (Eq.1) and its dissipation, ε (Eq. 2). ρk ( ρ uk i ) µ t k + = µ + + t xi xi Prk xi µ G ρε + S k, p (1) ρε ( ρuiε ) µ ε + = µ + + t xi xi Prε xi ε ( C1µ G C2 ρε) + Sε, p k where CB1B CB2B additional dimensionless model constants; PrBkB and PrBeB are the turbulent Prandtl numbers for kinetic energy and dissipation, respectively; SBk,pB and SBe,pB are source terms for the kinetic energy and turbulent dissipation; and the turbulent production rate (G) is defined in Eq. 3: u u i j ui 1 ρ ρ G = + 2 x j x i xj ρ xj xj (3) 2 ρk u i uj + 3 µ xj xj (2) Figure 6: Contours of velocity magnitude at x=0, -0.5 and -0.8 meter. Pathlines with velocity magnitude around the propeller are given in Figure 7. Where deformation of the pathlines due to the propeller can be clearly seen. It is also observed that velocity decreases after the propeller. 17. RESULS AND DISCUSSIONS Optimum geometry of the propeller was determined by lifting line method taking into account the diameter and revolution of the propeller, and power. Analyses were conducted by using called FLUEN. Lifting line method has been used to calculate the propeller thrust. he value obtained from lifting line was 19.8 kn. Propeller thrust was also calculated by CFD code of FLUEN as 19.2 kn. Results were obtained for 800 rpm for two different directions. Besides the propeller thrust, pressure, pathlines and velocity contours were obtained from FLUEN as given in the following figures. Contours of velocity magnitude for the propeller which has diameter of 0.8 m are shown in Figure 6 for n=800 rpm. In this figure contours are presented for three different streamwise directions, x=0 (at propeller blade), x=-0.5 and x=-0.8m. he deformation of the contours occurs in the space between the propeller and the exit, Maximum velocity value is determined at the propeller blades. Figure 7: Pathlines colored with velocity magnitude around the propeller. Velocity magnitude on the propeller is shown in Figure 8. Maximum velocity is occurred on the tip of the blades, while the minimum values are seen at the core of the propeller. As can be noted that differences between design and analyses tool are very small, therefore no modification was carried out in bow thruster geometry. Otherwise the propeller geometry, especially pitch distribution would be altered to have maximum thrust with the same power.

5 P Symposium P P symposium 6. Qing, L, Validation of RANS Prediction of Open Water Performance of a Highly Skewed Propeller with Experiments, Conference on Global Chinese Scholars on Hydrodynamics, Proceedings of the Conference of Global Chinese Scholars on Hydrodynamics July 2006, Pages Celik, F., A Numerical Study for Effectiveness of a Wake Equalizing Duct, Ocean Engineering , Celik, F., Guner, M., Energy Saving Device of Stator for Marine Propellers, Ocean Engineering 34, , Figure 8: Contours of velocity magnitude (m/s). 4. CONCLUSIONS In this work, a procedure is given to design and analyse of a bow thruster propeller. A real application is applied and results are shown. Overall it can be shown that this procedure can be applied a bow thruster propeller for depict an analysis, before manufacture. 5. REFERENCES 1. Naval Architecture Handbook, Editor: amer Yılmaz, published by the Chamber of urkish Naval Architects and Marine Engineers, İstanbul, 2008 (in urkish). 2. Bayraktar, S., Ozdemir, Y.H., Yılmaz,., nd Computational Investigation of a Hull, 2P International Conference on Marine Research and ransportation (ICMR 07), June 2007, Naples, Italy. 3. Abdel-Maksoud, M., Heinke, H.J., Scale Effects on th Ducted Propellers, 24P on Naval Hydrodynamics, July 2002, Fukuoka, Japan. 4. Hoffmann, P., Jaworski, S., Kozlowska, A., Comparison of EFD and CFD Investigations of Velocity Fields for Selected Body-Propeller Configurations, Archives of Civil and Mechanical Engineering, vol.7, no.3, Subramanian, V., Vijayakumar, R., An Inverse Design Approach for Minimising Wake at Propeller Plane Using CFD, Ocean Engineering 33, , Gu, H., Kinnas, S.A., Modeling of Contra-Rotating and Ducted Propellers via Coupling of a Vortex- Lattice with a Finite Volume Method, Propellers/Shafting 2003 Symposium, Society of Naval Architects and Marine Engineers, Virginia Beach, VA, September 17-18, Stoye,., Werner, K., ellkamp, J., Krüger, S., Abels, W., Load Generation Strategy for Propeller th Induced Pressure Fluctuations, 9P on Practical Design of Ships and Other Floating Structures, 2004, Lueneck-ravemuende, Germany. 11. Sileo, L., Bonfiglioli, A., Magi, V., RANSEs Simulation of the Flow Past a Marine Propeller Under Design and Off-Design Conditions 4th annual conference of the Computational Fluid Dynamics Society of Canada. 6. AUHORS BIOGRAPHY Yavuz Hakan Ozdemir has been working as research assistant since 2006 at YU department of Naval Architecture and Marine Engineering. His main research interests are computational fluid dynamics Seyfettin Bayraktar has been working as research assistant since 2000 at YU department of Naval Architecture and Marine Engineering. He is working on the last part of his PhD thesis. His main research interests are computational and experimental fluid dynamics. amer Yılmaz has been working as a lecturer since 1992 at YU, department of. Naval Architecture and Marine Engineering. His main interests are CFD and ship hydrodynamics Mesut Güner has been working as a lecturer since 1995 at YU, department of. Naval Architecture and Marine Engineering. His main interests are marine propulsors.